Respiratory produce (fruit, vegetables and plants) are commonly stored at a low temperature (typically close to 0° C.) in combination with a reduced O2 and increased CO2 partial pressure (so-called “Controlled Atmosphere (CA) storage”) to reduce their respiration rate, and, hence, extend their storage life. However, the optimal gas composition is critical, as too low an O2 partial pressure in combination with too high a CO2 partial pressure induces a fermentative metabolism in the fruit (Beaudry, Postharvest Biol Technol, 15: 293-303, 1999). This causes off-flavours (e.g., ethanol) and storage disorders (e.g., browning and core breakdown). For this reason, the O2 and CO2 partial pressure in commercial cool stores is kept at a safe and steady value. Such systems have been developed under U.S. Pat. No. 5,333,394, “Controlled atmosphere container system for perishable products”, U.S. Pat. No. 6,092,430, “Oxygen/carbon dioxide sensor and controller for a refrigerated controlled atmosphere shipping container” and U.S. Pat. No. 6,615,908, “Method of transporting or storing perishable product”. These patents dealt with atmosphere control, use of membranes and use of sensors. US Patent application US2007/0144638 was positioned as an improvement over these systems, being more economical (energy efficient) and not resulting in increased pressure in the containers (due to the regulation of the gases of the then current methods). European Patent EPO457431 describes a system for controlling oxygen and carbon dioxide concentrations in a refrigerated container for respiring perishables to dynamically and continuously control the gas concentrations. European Patent application EP2092831 describes a similar system. All these methods aim at obtaining predetermined values of gas concentrations. U.S. Pat. No. 5,333,394 describes a CA container with a controller that will implement bursts of gas supply which are preprogrammed based upon a particular application; it does not use measured gas production and consumption rates.
Further, U.S. Pat. No. 7,208,187 discloses a control method of a controlled atmosphere where at least one trace gas in a concentration of less than 1% is measured at least at two different times, and where the control variables are determined on the rate of change in the concentration of the trace gas, which is then used as a measure of the production rate of the trace gas. The referred gasses are ethylene, ethanol, ethane, acetaldehyde and carbon dioxide. The method does not consider gasses that are consumed due to respiration, i.e. oxygen. The method does also not consider the proportion of the rate of change of two gasses as a measure of physiological state.
Conventionally, controlled atmosphere (CA) storage of respiratory produce thus uses static, fixed set-points that are recommended as optimal storage conditions. As the concentrations are set at safe levels, significant firmness loss may still occur. In addition, the development of postharvest disorders, even under optimal CA, has been reported (Peppelenbos & Oosterhaven, Acta Hort 464: 381-386, 1998; DeLong et al., Acta Hort 737: 31-37, 2007). Due to the high biological variability of horticultural products the recommended optimal storage conditions may be different from the real optimal storage condition (Saltveit, Postharvest Biol Technol 27: 3-13, 2003; Veltman et al., Postharvest Biol Technol 27: 79-86, 2003).
Adaptive CA (ACA) storage systems can adapt the atmospheric gas composition based on the actual physiological state of the fruit (Veltman et al., Postharvest Biol Technol 27: 79-86, 2003; Zanella et al., Acta Hort 796, 77-82, 2008) as a function of fruit batch and time, such that variations due to factors such as geographical location, cultivar, mutant, orchard effects, harvest date and storage duration, can be taken into account. ACA storage can maintain fruit quality to a greater extent than conventional CA and Ultra low oxygen (ULO) storage facilities (Gasser et al., Acta Hort 796 69-76: 2008; Zanella et al., Acta Hort 796, 77-82, 2008), and has been proposed as a viable option for organic apple producers who are not using preventive chemicals (DeLong et al., Acta Hort 737: 31-37, 2007). Veltman et al. Postharvest Biol Technol 27: 79-86 (2003) showed that ACA resulted in quality improvement of ‘Elstar’ apples, with better firmness retention and inhibition of the ‘skin spots’ defect. Other applications of ACA have been successful for storage of apple cultivars ‘Granny Smith’ and ‘Delicious’ (Hoehn et al., In (M. M. Yahia): Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities, CRC Press, 42, 2009).
Monitoring systems for ACA have been developed based on chlorophyll fluorescence (Prange et al., international patent application N° WO02/06795) and monitoring the release of acetaldehyde or ethanol (Veltman et al., 2003).
The principle behind ACA storage is storage of fruit in an atmosphere with the lowest possible oxygen level that is tolerated by the fruit. Below this level fermentation becomes important and physiological disorders such as internal browning may develop. In practice, a fruit response signal which is generated under such conditions is used for monitoring oxygen stress. Two systems are already in use. Systems using chlorophyll fluorescence as the fruit response signal have been disclosed in International patent application WO02/06795. Controlled Atmosphere (CA) using chlorophyll fluorescence requires several expensive sensors per cool room, and has methodological constraints such as measurement position (a constant distance of the sensor to apples is required). Veltman et al. Postharvest Biol Technol 27: 79-86 (2003) used fermentative ethanol production as the fruit response signal. Ethanol measurements are conducted off-line in sampled fruits from the storage room or from the room air. The ethanol based system is disclosed in international patent application WO02/06795 and European patent EP0798962. The former method is a procedure that does not match the characteristics of a dynamic commodity indicator as part of an automated control system. The latter is unreliable due to possible interaction of the detection equipment with gasses such as ethylene, which may be present in the sample air (Hoehn et al., In (M. M. Yahia): Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities, CRC Press, 42, 2009).
Both methods have been benchmarked against respiration measurements. The onset of stressful conditions indicated by increased ethanol concentration or chlorophyll fluorescence signal concurs with the lowest acceptable respiration rate, which can be obtained by measuring the changes of O2 oxygen and/or CO2 concentration in the atmosphere around the fruit (Veltman et al. Postharvest Biol Technol 27: 79-86, 2003; Gasser et al., Acta Hort 796: 69-76, 2008). Measurement of respiration rate in storage rooms has not been found practical (Hoehn et al., In (M. M. Yahia): Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities, CRC Press, 42, 2009).
Yearsly et al., Postharvest Biol Technol 8: 95-109 (1996) and Gasser et al., Acta Hort 796: 69-76 (2008) demonstrated on small batches of apples in jars that the respiration coefficient RQ (rate of CO2 production per rate of consumption of O2) increases drastically below the lowest respiration rate, due to the onset of fermentation. This demonstrated that RQ concurs with chlorophyll fluorescence and ethanol methods.
Jar experiments in the laboratory however exclude important influencing factors of actual storage rooms (size and shape of the room, leakages, climate conditions, stacking pattern, storage of gasses inside the fruits) that prevent exact determination of RQ, and therefore make accurate control in real systems impossible. In particular, jars provide an air-tight system that excludes leakages and can be controlled to prevent temperature and pressure fluctuations.
In containers, chambers or rooms, accurately determining gas leakage rates is essential for correcting measurements of physiological processes such as respiration and fermentation (Hoehn et al., In (M. M. Yahia): Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities, CRC Press, 42, 2009). Existing methods use non-reacting tracer gasses for this purpose in separate tests (Baker et al., Environ Exp Bot 51: 103-110, 2004) or pressure decay in empty rooms at ambient temperature (Bartsch, Cornell Fruit Handling and Storage Newsletter, 16-20, 2004; Raghaven et al., In (D. M. Barett, L. Somogy, H. Ramaswamy): Processing Fruits, CRC Press, 23-52, 2005). Leakage is dependent on room design and construction, climate conditions, load, and changes with time. Yearly tests are recommended (Hoehn et al., In (M. M. Yahia): Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities, CRC Press, 42, 2009).
There is still need for improvement of storage rooms and control systems thereof.